1. Field of the Invention
This invention relates to an improved catalytic hydrogenation process for the conversion or upgrading of carbonaceous materials that are substantially solid at room temperature to hydrogen-enriched hydrocarbonaceous liquid products and further relates to an improved solvent extraction process for making liquid fuels from coal. More particularly, this invention relates to an improved process for converting solvent refined coal bottoms to distillable liquids.
2. Description of the Prior Art
Solvent extraction processes for converting coal to clean liquid or solid fuels generally comprises the following steps:
(1) coal extraction; PA1 (2) separation of extract from undissolved residue; and PA1 (3) hydrogenation of the separated extract. PA1 (1) primary catalytic hydrocracking of extract to produce high boiling distillable liquids; PA1 (2) catalytic hydrofining or hydrotreating of the high boiling distillable liquids, and PA1 (3) secondary catalytic hydrocracking of the hydrofined high boiling distillate liquids to produce liquid fuels. PA1 (1) separating light gases containing hydrogen, CO.sub.2, H.sub.2 S, NH.sub.3, and light, C.sub.2 -C.sub.4 hydrocarbons; PA1 (2) separating a C.sub.5 - 300.degree. F cut and hydroconverting it in a hydrotreater; PA1 (3) separating a 300.degree.-750.degree. F gas oil cut and hydroconverting it by treatment in a hydrotreater followed by further treatment in a hydrocracker; PA1 (4) separating a 750.degree.-1050.degree. F heavy gas oil cut and hydroconverting it in a hydrocracker; and PA1 (5) separating a 1050.degree. F bottoms cut containing ash, heavy hydrogen-deficient carbonaceous material, and very-difficult-to-extract nonhydrogen-deficient hydrocarbons, and coking it in the presence of a hydrogen-donor gas oil derived from the hydrocracked heavy gas oil cut (item 4, supra). PA1 (1) a 450.degree.-600.degree. F naphtha and heating oil fraction; PA1 (2) a 600.degree.-950.degree. F heavy gas oil fraction which is employed in it as recycle solvent, the remainder of which is further treated in a catalytic hydrocracking zone; and PA1 (3) a 950.degree. F+ heavy vacuum bottoms fraction which is either employed directly as fuel or catalytically hydrocracked.
In the coal extraction step, coal is slurried in an organic solvent and heated, often in the presence of added molecular hydrogen, to a temperature sufficient to dissolve or liquefy most of the organic material in the coal. A wide variety of solvents are known in the art, including hydrogen donor solvents, non-hydrogen donor solvents, and mixtures thereof. The solvent employed may be obtained from a previous conversion of coal or may be a solvent generated externally of the solvent extraction process or may be mixtures thereof. Thus, the solvent may be the liquid wherein the liquid product obtained by solvent extraction is dissolved as well as the liquid in which the undissolved solids from the coal extraction are dispersed.
After extraction, solids that are present may be removed from the product stream. The product stream, separated from undissolved residue, may then be stripped of solvent and the solvent recycled to the extraction step. The remaining extract, which will herein be referred to as "solvent refined coal," is a solid at room temperature and contains very little (generally less than about 10 weight percent) material boiling below 850.degree. F. The solvent refined coal (which may still contain some "solvent") may thereafter be subjected to distillation to obtain products of various boiling ranges, some of which are useful as fuels. These fractions may then be further treated by refining processes including coking, hydrogenation, cracking, hydrocracking, hydrotreating, and the like.
Alternatively, the product stream containing the solvent refined coal may be subjected to further treatment without first removing the solvent. It is known that such processing schemes have the advantage of treating an easier-to-handle material than those in which the solvent is separated before further treatment. Again, the mixture containing solvent refined coal may be further treated by fractionation, coking, hydrogenation, cracking, hydrocracking, and the like.
Generally, further treatment of the separated coal extract may be described as hydrogenation, as has been noted above. Hydrogenation of coal extract or solvent refined coal usually occurs in three steps (see U.S. Pat. No. 3,523,886, for example):
The function of the primary catalytic hydrocracking zone is to convert at least a portion of the nondistillable coal extract or solvent refined coal to an ash-free, distillable hydrocarbonaceous liquid boiling below about 750.degree. to 950.degree. F. However, the catalytic hydrogenation conditions are not sufficiently severe to yield a distillable product which is free of heteroatomic nitrogen, oxygen, and sulfur compounds or to yield a distillable product of which major portion boils in the gasoline boiling range. The function of the catalytic hydrofining or hydrotreating zone is to remove substantially all of the heteroatomic contaminants from all or the lower boiling fractions of the ash-free, distillable hydrocarbonaceous liquids. The function of the secondary catalytic hydrocracking zone is to lower the boiling range of at least the higher boiling fraction of the hydrofiner effluent to produce a liquid boiling in the gasoline boiling range.
Treatment of the higher boiling fractions of solvent refined coal and the undissolved residue often includes a carbonization or thermal cracking step. For example, U.S. Pat. No. 3,143,489 teaches a solvent extraction process wherein coal is slurried in an extraction zone with a suitable solvent to effect partial conversion of the coal, the liquid extract is separated from the undissolved residue, the residue is further processed in a carbonization zone, and the liquid products are subjected to catalytic hydrogenation in succeeding zones. (See also U.S. Pat. No. 3,523,886.)
U.S. Pat. No. 3,518,182 discloses a process wherein coal is solvent extracted in the presence of hydrogen, the extract (or the hydrovisbreaker effluent, as it is referred to in the '182 specification) is fractionated, and the various fractions are subjected to subsequent coking or hydrogenation steps to produce motor fuels from coal with the object of minimizing hydrogen consumption. More particularly, '182 teaches fractionating the solvent refined coal/solvent mixture and treating the respective fractions as follows:
A process similar to that of '182 is disclosed in U.S. Pat. No. 3,607,718, the primary difference residing in the fact that '718 teaches removal of undissolved residue prior to fractionation of the solvent refined coal/solvent mixture and further teaches direct recycling of a 950.degree. F+ bottoms fraction to the coal extraction step.
Catalytic hydrogenation of the higher boiling fractions and the undissolved residue is also known in the art. For example, U.S. Pat. No. 3,488,279 discloses a solvent extraction process wherein coal is first extracted by a hydrogen-donor solvent, at least a portion of the solvent and the liquid products of the hydrogen-donor extraction is then separated from the undissolved residue, and the residue is further treated in a catalytic hydrogenation zone. The combined liquid products from the hydrogen-donor extraction and the catalytic hydrogenation step are then distilled to yield the following fractions:
A problem encountered in the hydrogenation of solvent refined coal is the viscous nature of the coal extract, particularly the heavier fractions of solvent refined coal which contains a principal portion of the total coal extract. Catalytic hydrogenation is even more difficult because sufficient catalyst/reactant contact is not easily obtained. The use of carrier solvents to enhance the flowability of the solvent refined coal and thereby enhance catalyst/reactant contact is known. However, the prior art processes do not selectively treat the heavier solvent refined coal fractions. Therefore, excessive cracking of the carrier solvent occurs. Not only does this reaction produce less valuable materials, but it also causes excessive hydrogen consumption. Accordingly, an improved method of converting higher-boiling fractions of solvent refined coal to distillable or pumpable liquids is needed.